2 Answers

3 votes

2.
>V Black and White Opelucid City Meet Professor Juniper in town after earning all eight Unova Badges.
V Black and White Castelia City Pokémon Center Talk to a man after the player has outsider Pokémon with at least 50 different original Trainers. The man will no longer give out gifts after the Master Ball.

I know you probably want to have master balls in other gen. 5 games, but if you trade them when attached to a Pokemon, get one after collecting 8 gym badges, and trading with 50 people then talking to the guy in Castelia City, you could get at most, 6 master balls in one game (Assuming you have both White and Black). (With Black 2 and White 2, Join Avenue's Raffle Shop may let you get multiple Master Balls.)